A vibrator is a device which is used to form vibrations transmitted into the earth in seismic investigations. A vibrator is a device that is typically installed at the earth's surface to form a vibration which is directed into the earth and reflected by various geological horizons in the earth to thereby provide data indicative of the nature of the seismic formations. The propagation of the vibrations into the earth involves a propagation of a generally spherical wavefront. Thus, the intensity of the energy on a per unit area basis decreases with the square of the distance of the propagation. Since the signal must be propagated into the earth, reflected at a far horizon and return to the surface, the reflected signal is also subject to a square law decrease, the propagation of the signal involves a decrease in intensity which is a function of the fourth power of the distance. This can be reduced significantly by installing a vibrator in a well borehole. Such a vibrator has to be an elongate cylindrical structure to fit in the well borehole. Devices of this sort typically are hydraulically powered. A vibrator therefore is required which converts hydraulic power into vibrations for surface and down hole uses.
The present disclosure is directed to such a system which enables the extension of the frequency range of the device. Using a typical low frequency of 10 hertz as an example, it is desirable that the frequency increase up to some significantly greater level for operation. The response of the seismic formations is different, and therefore a change in frequency provides added information with regard to the formations. The present disclosure is directed to control valves which are connected to provide hydraulic power to both sides of a piston which is in a cylinder to enable the piston to be driven in a double acting mode. The cylinder and piston construction reciprocates a reactionary mass.
Consider as an example a device which operates at 10 hertz. If it is desirable to operate the same vibrator at 600 hertz and output the same force, the system must change the stroke by an amount which is very substantial. An example will be developed below where the stroke at 10 hertz is 1.9 inches while the stroke is reduced at 600 hertz to 0.0005 inches. Focusing for the moment on the power requirements for that, the performance of the hydraulic fluid becomes a significant limiting factor. In other words, the hydraulic oil which is normally assumed to be an incompressible liquid at most circumstances limits the performance. The fact that it is somewhat compressible is dependent primarily on the bulk modulus of the hydraulic oil and the volume of the hydraulic oil that is in the cylinder. The first factor can be enhanced by selection of hydraulic fluids which have the optimum bulk modulus. The last factor more aptly involves the design of the flow lines and the filled region of the cylinder. In other words, the frequency response of the system can be increased if the volume inside the moveable parts is reduced. One way to reduce this is to decrease the volume inside the flow lines. Another way to reduce this volume is to reduce the flow regions of the valves which connect the system. In other words, the volume inside the cylinder is reduced. As will be described in some detail, the present disclosure omits a four-way valve and uses instead two separate three-way valves. The three-way valves are located at opposite ends of the cylinder. Moreover, the three-way valves are immediately adjacent to the ends of the pistons in the cylinder for suitable switching. Furthermore, for operation at high frequency, the cylinder length can be shortened so that the cylinder chambers are reduced thereby accommodating extending performance at higher frequencies. This enables a system whereby the reciprocating mass is located near the center of the chamber within the cylinder. This enables the extended frequency performance mentioned. The present disclosure further sets forth a mechanism whereby the head area can be extended. Through this technique, there is an enhanced head region so that low frequencies can be generated by involving the added head region. It it not involved for operation at high frequencies. In other words, this reduces the volume of the head area so that hydraulic fluid requirements are reduced at high frequencies.
The present apparatus is a wide frequency range, sweep frequency vibrator of elongate cylindrical construction which especially finds use in downhole seismic generators. It is a system which is hydraulically powered where the hydraulic fluid is delivered through two separated three-way valves. They are located at opposite ends of the structure. Primarily, the structure involves an elongate cylindrical chamber or housing which is closed at the ends. There is a vibrating mass which is driven in a double acting mode. As will be detailed, it is driven at different frequencies depending on the switching of the hydraulic fluid which is delivered through the valves. Examples of operation will be given.